Polyurethane integral skin foams of the type obtained by the method of foaming in a mold (see e.g. German Auslegeschriften 1,196,864, corresponding to British Pat. No. 969,114, and 1,694,138, corresponding to French Pat. No. 1,559,325 and British Pat. No. 1,209,243) are eminently suitable for the mass production of lightweight building construction (e.g. for the furniture, vehicle and house building industries) and of elastic molded articles (such as cushions, shock absorbers and shoe soles).
The molded products of flexible, semirigid and rigid polyurethane foams currently available are generally based on the reaction of aromatic polyisocyanates with polyether or polyester polyols, optionally in combination with chain lengthening agents and cross-linking agents. These molded articles have superior mechanical properties which render them suitable for many of the applications already mentioned.
A disadvantage of all these molded products is that they turn yellow due to weathering and degradation of the surface. Those molded products which are to be exposed to relatively stringent conditions must, therefore, generally be covered with a protective layer.
The discoloration of polyurethanes under the influence of light is known and is particularly severe when aromatic polyisocyanates are used. To alleviate this problem, the manufacture of polyurethane foams, with or without an integral skin, from aliphatic polyisocyanates has been pursued. The development of a suitable catalyst combination has often been the major objective of these experiments (see e.g. German Offenlegungsschrift 2,710,901). Although these foams are substantially more stable to light than those previously known, they still undergo discoloration after prolonged exposure to UV light necessitating the addition of antioxidants and UV adsorbers to insure the stability of these foams to light.
One distinct disadvantage of foams based on aliphatic polyisocyanates is their much higher susceptability to decomposition by heat than that of conventional polyurethanes produced from aromatic polyisocyanates. This also necessitates the addition of antioxidants or stabilizers.
It is, therefore, an object of the present invention to provide a process for the production of polyurethane foams, preferably having an integral skin, which will satisfy high standards of resistance to UV light as well as being sufficiently resistant to decomposition by heat. By "high standards of resistance to UV light" is meant that the material is substantially unchanged after at least 500 hours in the Xenon test (DIN draft 53 387; Kunststoff-Handbuch Vol. VII, by Vieweg and Hochtlen, Carl Hanser Verlag Munich, 1966, page 426). By "sufficient resistance to decomposition by heat" is meant that the material will withstand a storage time of 72 hours at 110.degree. C. without becoming unusable.